Time-lag fuses having high thermal efficiency



July 19, 1966 F. J. KOZACKA 3,251,950

LAG FUSES HAVING HIGH THERMAL EFFICIENCY TIME- Filed Nov. 30, 1964FIG.I.

INVENTOR= EMREDERICK J. KOZACKA,

BY WW ATTORNEY United States Patent 3,261,950 TIME-LAG FUSES HAVING HIGHTHERMAL EFFICIENCY Frederick J. Kozacka, South Hampton, N.H., assignorto The Chase-Shawmut Company, Newburyport, Mass. Filed Nov. 30, 1964,Ser. No. 414,750 8 Claims. (Cl. 200-120) This invention is concernedwith time-lag fuses, and more particularly time-lag fuses havingsufficiently long time-lags to preclude blowing on occurrence of heavystarting currents in motor circuits.

It is one object of this invention to provide improved electric fuses ofthe above description which have a high thermal efficiency.

It' is another object of this invention to provide improved electricfuses of the above description which minimize heat flow in the presenceof an arc-quenching filler, such as quartz sand having a high thermalconductivity.

Still another object of this invention is to provide timelag fuses ofthe above description avoiding the presence of any movablecircuit-interrupting mechanisms, having a high interrupting capacity andconcomitant currentlimiting action, and being of smaller size, or morecompact, than comparable prior art fuses.

A further object of the invention is to provide highinterruptingcapacity fuses including link-severing means which are predicated upon ametallurgical reaction be- .tween a metal having a relatively highfusing point and a metal having a relatively low fusing point, whichfuses include means particularly effective for minimizing thedissipation of the heat generated therein, thus requiring for any givencurrent rating fuse links having a relatively large mass andcross-sectional area, conductive to long time-lags.

A further object of the invention is to provide electric time-lag fuseswhose current path has a relatively low impedance and includes improvedmeans to oppose heat transfer away from the center region of the fusestructure in a direction longitudinally thereof.

These and other objects of the invention and advantages thereof willbecome more apparent from the accompanying drawings and the followingdescription of a preferred embodiment of the invention.

In the drawings;

FIG. 1 is a top-plan View of an electric fuse embodying this invention;

FIG. 2 is a section along 2 -2 of FIG. 1;

FIG. 3 is a top-plan view of a sheet metal blank used for forming thefuse link structure of FIGS. 1 and 2;

FIG. 4 is a top-plan view of the vblank shown in FIG. 3 upon beingprovided with a link-severing overlay and appropriately folded; and

FIG. 5 is a side elevation of the structure of FIG. 4.

Referring now to the drawings, and more particularly to FIGS. 1 and 2thereof, numeral 1 has been applied to indicate a tubular casing ofinsulating material closed on both ends by electroconductive terminalelements in the form of plugs 2, each provided with a blade contact 2afor insertion of the fuse structure into a fuse holder and an electriccircuit. Casing 1 is filled with a pulverulent arc-quenching medium orfiller 3, preferably quartz sand because of the high heat absorbingcapacity of the latter, resulting in generation of high arc-voltages onblowing of the fuse. Terminal plugs 2 are provided with holes 2ballowing casing 1 to be filled with the arcquenching medium 3. Theseholes 25 are normally closed by sheet metal caps, each coextensive withone of Patented July 19, 1966 the holes 2b. The axially inner surfacesof terminal plugs 2 are provided with a plurality of parallel grooves 20for receiving the axially outer ends of ribbon fuse links 4. Casing 1houses four ribbon fuse links 4 which are submersed in the arc-quenchingfiller 3. Fuse links 4 are made of a current-limiting metal. The termcurrentlirniting metal is used as a term encompassing silver and copperand alloys thereof having substantially the same physical properties(conductivity, fusing point, specific heat, fusing i -t, etc.) as silverand/or copper.

As clearly shown in FIG. 3 each fuse link 4 comprises an axially innerportion 4a and axially outer portion 4b. The axially inner portion 4a isprovided with three transverse lines R, S, T of circular perforations.Each line R, S, T of perforations forms a point of minimumcrosssectional area, or of minimum active width, respectively. Theaxially inner portion 4a has a larger cross-sectional area and a largerWidth than each of the axially outer portions 4b. The cross-sectionalarea and the width of the axially outer portions 4b exceeds thecross-sectional area and the width of the fuse links 4 at the pointsthereof where these two parameters are reduced by virtue of the lines R,S, T of perforations. The center line S of perforations is associatedwith a link-severing overlay 5 having a considerably lower fusing pointthan the base metal-silver or copper-of which links 4 are made. Theoverlay metal may be tin, or appropriate alloys of tin, or indium.

The regions of fuse links 4 adjacent the transverse lines R and T ofperforations are sandwiched between pairs of plates 6 of insulatingmaterial, preferably a syntheticresin-glass-cloth laminate, which arestapled together. Thus the regions of the fuse links 4 immediatelyadjacent the center lines S of perforations are exposed to the immediateaction of the arc-quenching filler 3, whereas the arc-quenching filler 3has no immediate access to the regions of the fuse links 4 adjacent thetransverse lines R and T of perforations. Consequently different kindsof arc voltages, or arc voltages having different rates of rise and ofdecay, are generated at the center lines S of perforations, on the onehand, and on the axially outer lines R and T of perforations, on theother hand, as more fully explained in United States Patent 2,964,604 toP. C. Jacobs, Jr. et al., December 13, 1960 for Current-Limiting FusesHaving Compound Arc Voltage Generating Means, assigned to the sameassignee as the present invention.

It will be noted that while the center portions 4a of fuse links 4 areperforated, the axially outer end portions or connector tabs 4b of fuselinks 4 are not perforated.

The radially i-n-ner fuse links 4 are sandwiched between the radiallyouter fuse links 4, and the latter thus form thermal barriers, orshields, minimizing the escape of heat in radial direction from theradially inner fuse links 4. The latter are heated mutually, and also bythe heat generated in the radially outer fuse links 4. Heat flow fromthe region of highest temperature adjacent the center lines S ofperforations is minimized by reason of the fact that the axially outerportion 4b of fuse links 4 are folded, or bent, to form loops 4csubstantially in the shape of the letter S, and substantially in theshape of the in- I verted letter S. The loops 40 shown to the left ofFIG. 2 are i n the shape of the letter S, and the loops 4c shown to theright of FIG. 2 are in the shape of the inverted letter S. In otherwords, the axially outer portions or connector tabs 4b of fuse links 4are folded in zig-zag fashion and form two serially related U-turns.

If the portions 4b of fuse links 4 were straight rather than folded andof equal length, this would result in a drastic increase of the heatflow away from the axially inner portions 4a of fuse links 4 in radialdirection and in axial direction. The loops 4c as provided radicallyreduce the heat flow away from the central portions 4:: of fuse links 4,but do not significantly contribute to the generation of heat since theyare not perforated, and since their cross-section or width is relativelylarge.

The cross-section or Width of the axially outer portions or connectortabs 4b of fuse links 4 must exceed the crosssection or width of thefuse links 4 at the points thereof Where their cross-section or activewidth is minimal on account of the presence of transverse lines R, S, Tof perforations in order to compel arc initiation at the axially innerportion 4a of fuse link 4 and to preclude arc initiation at any point ofthe axially outer portions or connector tabs 4b of fuse links 4. Thelatter would be dangerous because it might result in back-burning of theare into terminal plugs 2.

It will be apparent from FIG. 3 that the axially inner portions 4a formshoulders 4d at the points of transition to the radially outer portionsor connector tabs 4b.

The axially outer ends of the axially outer portions or connector tabs41; are inserted into the grooves 2c in terminal plugs 2 andconductively connected to the latter by solder joints formed inside ofsaid grooves.

The structure of FIGS. 1 and 2 achieves long timelags or time delays bythe coaction of the following factors:

(a) Since the fusing point of the overlay metal is low, the temperaturerequired at the centers of links 4 to cause the links to be severed by ametallurgical reaction between their base metal and their overlay metalis low.

(b) Since the sandwiching of radially inner fuse link portions byradially outer fuse link portions greatly reduces heat transfer awayfrom the radially inner fuse link portions 411 of fuse link 4,relatively little heat generation is required to bring the overlay metalto the relatively low fusing point and to cause the links 4 to besevered by a metallurgical reaction.

(c) Since the serially related U-turns 4c of links 4 result in a furtherdrastic reduction of heat dissipation from the axially inner portion 4aof fuse links 4, the heat generation, or watt losses, required to bringthe linksevering overlays to their fusing point is brought to anunprecedented low. In other words, considering a fuse structure as shownin FIGS. 1 and 2 having given watt losses, such a fuse structure tendsto have but an extremely small minimum fusing current on account of itshigh thermal efficiency, i.e. on account of its small heat losses. Inorder to impart to the fuse structure higher minimum fusing currents, ora larger current-carrying capacity, respectively, the mass andcross-section of the axially inner portions 4a and that of the axiallyouter portions 4b must be significantly increased, resulting in asignificant increase of heat storage capacity. This increase, in turn,results in a significant increase of time-lag, or time delay.

In the embodiment of the invention shown, the portions 4a and 4b of allfour fuse links 4 are arranged in spaced parallel planes. Each of loops4c encompasses three spaced planes and a separate groove 20 is providedin each plug 2 for each end of each fuse link. If desired, the number ofgrooves 2c may be reduced and each groove may receive the ends of twofuse links 4.

The structure shown in FIGS. 1 and 2 makes it possible to achievetime-lags, or time delays, in the order of to seconds at five times therated current, though quartz sand is being used as arc-quenching mediumin order to achieve reasonably high arc-voltages with a relatively smallnumber of series breaks.

The structure shown in FIGS. 1 and 2 is similar to that shown in UnitedStates Patent 3,123,693 to Frederick J. Kozacka, March 3, 1964, Time-LagFuses of the Blade Contact Type, assigned to the same assignee as thepresent invention, inasmuch as both aim to achieve a high thermalefficiency by the provision of radially outer fuse link means formingthermal shields for radially inner fuse link means. However, in thestructure of FIGS. 1 and 2 afar greater thermal efilciency is achievedby virtue of the presence of the axially outer fuse link portions orconnector tabs 4b and the fashion in which these fuse link portions orconnector tabs are folded.

The high thermal elficiency which is characteristic of the structuredisclosed makes it possible .to greatly reduce the physical dimensionsthereof relative to those of prior art fuse structures having comparabletime-current curves and having an arc-quenching filler such as quartzsand combining high arc-quenching action with high thermal conductivity.

It will be apparent to those skilled in the art that various changes andmodifications may be made without departing from the spirit of theinvention as set forth in the appended claims.

I claim as my invention:

1. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of said casing;

(c) a pulverulent arc-quenching medium inside said casing; and

(d) a plurality of ribbon fuse link means of a currentlimiting metalinside said casing submersed in said medium conductively interconnectingsaid pair of terminal elements, said plurality of fuse link meansincluding radially inner fuse link means and radially outer fuse linkmeans sandwiching said radially inner fuse link means, said fuse linkmeans having axially inner portions provided with transverse lines ofperforations defining points of minimum crosssectional areasubstantially less than the maximum cross-sectional area of said axiallyinner portions, said axially inner portions supporting link-severingoverlays of a metal having a relatively low fusing point, said fuse linkmeans including axially outer portions having a cross-sectional arealarger than said minimum cross-sectional area but less than said maximumcross-sectional area, said axially outer portions being folded to formloops substantially in the shape of the letter S and substantially inthe shape of the inverted letter S.

2. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of the casing;

(c) a pulverulent arc-quenching medium inside said casing; and

(d) a plurality of ribbon fuse link means of a currentlimiting metalinside said casing conductively interconnecting said pair of terminalelements, said plurality of fuse link means including axially innerperforated portions arranged in substantially parallel spaced planeseach supporting an overlay of a link severing relatively low fusingpoint metal, said fuse link means including axially outer connector tabshaving a width smaller than said axially inner portions, said axiallyinner portions forming shoulders at the points of transition to saidconnector tabs, and each of said connector tabs including a zig-zagsection situated between one of said axially inner portions and one ofsaid pair of terminal elements.

3. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of said casing;

(c) a pulverulent arc-quenching filler inside said casing; and

(d) a plurality of ribbon fuse link means of a currentlimiting metalinside said casing submersed in said filler conductively interconnectingsaid pair of terminal elements, said fuse link means including axiallyinner perforated portions arranged in substantially parallel spacedplanes each supporting an overlay of a link-severing relatively lowfusing point metal, said fuse link means further includingnon-perforated connector tabs on the axially outer ends thereof having asmaller Width than said axially inner portions, said axially innerportions forming shoulders at the points of transition to said connectortabs, and each of said connector tabs forming two serially relatedU-turns.

4. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of said casing;

(c) a pulverulent arc-quenching medium inside said casing;

(d) a plurality of ribbon fuse link means of a currentlimiting metalinside said casing submersed in said medium conductively interconnectingsaid pair of terminal elements, said plurality of fuse link meansincluding first fuse link means arranged relatively close to the axis ofsaid casing and second fuse link means arranged relatively remote fromthe axis of said casing sandwiching said first fuse link means, each ofsaid plurality of fuse link means including an axially inner perforatedportion having a predetermined maxi-mum Width and supporting alinksevering overlay of a metal having a relatively low fusing point,each of said plurality of fuse link means further including a pair ofconnector tabs having a width less than said maximum Width of saidaxially inner portions, said axially inner portions forming shoulders atthe points of transition thereof to said connector tabs, and saidconnector tabs being folded to form loops encompassing three spacedplanes and being substantially in the shape of the letter S andsubstantially in the shape of the inverted letter S.

5. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of metal plugs each closing one end of said casing, each ofsaid pair of plugs having parallel grooves on the axially inner endsurface thereof;

(c) a quartz sand filler arranged inside said casing; and

(d) four ribbon fuse links of a current-limiting metal inside saidcasing submersed in said quartz sand filler, each of said four fuselinks having axially outer ends inserted into said grooves in each ofsaid pair of plugs, each of said four fuse links including an axiallyinner portion having transverse lines of perforations defining points ofpredetermined minimum cross-sectional area substantially less than themaximum cross-sectional area of said axially inner portions of said fuselinks, said axially inner portion of each of said fuse links supportinga link severing overlay of a metal having a relatively low fusing point,said four fuse links further including axially outer non-perforatedportions having a crosssectional area larger than said points of minimumcross-sectional area but less than said maximum cross-sectional area ofsaid axially inner portion of said fuse links, and said axially outerportions being folded to form loops substantially in the shape of theletter S and substantially in the shape of the inverted letter S.

6. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of said casing;

(c) a pulverulent arc-quenching filler inside said cas- (d) fusibleribbon conductor means of a current-limiting metal inside said casingsubmersed in said filler conductively interconnecting said pair ofterminal elements, said fusible conductor means being provided withtransverse lines of perforations and supporting link-severing overlaymeans of a metal having a relatively low fusing point;

(e) additional fusible ribbon conductor means of a current-limitingmetal inside said casing submersed in said filler conductivelyinterconnecting said pair of terminal elements, said additional fusibleconductor means being provided with transverse lines of perforations andsupporting link-severing means of a metal having a relatively low fusingpoint, said additional fusible conductor means overlapping at least inpart said conductor means to minimize substantially radial heat flowfrom said conductor means to said casing; and

(f) axially outer non-perforated connector tabs integral with saidfusible conductor means and being folded to form adjacent to each ofsaid pair of terminal elements, a pair of serially related U-turns ofsufiicient size to significantly limit axial heat flow from the centerregion of said fusible conductor means to said pair of terminalelements.

7. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of said casing;

(c) a pulverulent arc-quenching filler inside said cas- (d) fusibleribbon conductor means of a current limiting metal inside said casingsubmersed in said filler conductively interconnecting said pair ofterminal elements, said fusible conductor means being provided withtransverse lines of perforations and supporting link-severing overlaymeans of a metal having a relatively low fusing point;

(e) additional fusible ribbon conductor means of a current-limitingmetal inside said casing submersed in said filler conductivelyinterconnecting said pair of terminal elements, said additional fusibleconductor means being provided with transverse lines of perforations andsupporting link-severing means of a metal having a relatively low fusingpoint, said additional fusible conductor means overlapping at least inpart said conductor means to minimize substantially radial heat flowfrom said conductor means to said casing; and

(f) axially outer non-perforated connector tabs integral with saidadditional fusible connector means, and being folded to form adjacent toeach of said pair of terminal elements a pair of serially related U-turns of sufficient size to significantly limit axial heat flow from thecenter region of said fusible conductor means to said pair of terminalelements.

8. An electric time-lag fuse comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of electroconductive terminal elements each arranged at oneof the ends of said casing;

(c) a quartz sand filler inside said casing;

(d) a pair of substantially parallel ribbon fuse links of acurrent-limiting metal inside said casing submersed in said quartz sandconductively interconnecting said pair of terminals, said pair of fuselinks being provided with transverse lines of perforations andsupporting link-severing overlays of a metal having a relatively lowfusing point;

(e) additional ribbon-shaped fusible conductors of a current-limitingmetal inside said casing submersed in said squartz sand shunting saidpair of fuse links, each of said pair of additional fusible conductorsoverlapping at least in part one of said pair of fuse links beingprovided with transverse lines of perforations and supportinglink-severing overlays of a metal having a relatively low fusing point;and

(f) pairs of axially outer non-perforated connector tabs each integralwith one of said pair of fuse links, each connector tab of said pairs ofconnector tabs being folded to form adjacent of each of said pair ofterminal elements a pair of serially related U- turns of sufficient sizeto significantly limit axial heat flow from said pair of fuse links tosaid pair of terminal elements.

No references cited.

BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Examiner.

1. AN ELECTRIC TIME-LAG FUSE COMPRISING IN COMBINATION: (A) A TUBULARCASING OF INSULATING MATERIAL; (B) A PAIR OF ELECTROCONDUCTIVE TERMINALELEMENTS EACH ARRANGED AT ONE OF THE ENDS OF SAID CASING; (C) APULVERULENT ARC-QUENCHING MEDIUM INSIDE SAID CASING; AND (D) A PLURALITYOF RIBBON FUSE LINK MEANS OF A CURRENTLIMITING METAL INSIDE SAID CASINGSUBMERSED IN SAID MEDIUM CONDUCTIVELY INTERCONNECTING SAID PAIR OFTERMINAL ELEMENTS, SAID PLURALITY OF FUSE LINK MEANS INCLUDING RADIALLYINNER FUSE LINE MEANS AND RADIALLY OUTER FUSE LINK MEANS SANDWICHINGSAID RADIALLY INNER FUSE LINK MEANS, SAID FUSE LINK MEANS HAVING AXIALLYINNER PORTIONS PROVIDED WITH TRANSVERSE LINES OF PERFORATIONS DEFININGPOINTS OF MINIMUM CROSSSECTIONAL AREA SUBSTANTIALLY LESS THAN THEMAXIMUM CROSS-SECTIONAL AREA OF SAID AXIALLY INNER PORTIONS, SAIDAXIALLY INNER PORTIONS SUPPORTING LINK-SEVERING OVERLAYS OF A METALHAVING A RELATIVELY LOW FUSING POINT, SAID FUSE LINK MEANS INCLUDINGAXIALLY OUTER PORTIONS HAVING A CROSS-SECTIONAL AREA LARGER THAN SAIDMINIMUM CROSS-SECTIONAL AREA BUT LESS THAN SAID MAXIMUM CROSS-SECTIONALAREA, SAID AXIALLY OUTER PORTIONS BEING FOLDED TO FORM LOOPSSUBSTANTIALLY IN THE SHAPE OF THE LETTER S AND SUBSTANTIALLY IN THESHAPE OF THE INVERTED LETTER S.